Adhesion of invasive cancer cells to vascular endothelium is a critical first and selective step in metastasis. During the last few years a large number of different adhesion molecules have been discovered to be involved in cell migration and homing mechanisms of hemopoietic cells. Transformed tumor cells can abuse such mechanisms in an uncontrolled way leading to metastasis in tissues other than those of their origin. The antigen known as .alpha.6-integrin is an adhesion molecule involved in metastasis.
Carcinoma, melanoma, and endothelial cells are known to adhere to laminin, an extracellular matrix molecule. Several integrin complexes, including .alpha.1/.beta.1, .alpha.2/.beta.1, and .alpha.6/.beta.1 help mediate this binding. .alpha.6/.beta.1 appears to react with laminin monospecifically.
Three antibodies directed against the .alpha.6 integrin chain have been described in the literature, each produced in a different manner. Antibody GoH3 was produced by immunizing rats with blood platelets and recognizes the platelet protein called complex Ic-Ila, which was subsequently defined as .alpha.6/.beta.1 integrin. This antibody blocks cell-laminin interaction, and defines .alpha.6 integrin as a receptor for a fragment called E8 obtained by an elastase digest of laminin. [Sonnenberg et al., J. Biol. Chem. 262, 10376-10383 (1987); Hemler et al., J. Biol. Chem. 263, 7660-7665 (1988)]. Antibody 135-13C was prepared in rats against purified tumor associated proteins TSP-180, which is now described as .alpha.6/.beta.4 integrin. This antibody has a less pronounced effect on cells binding to laminin. [Kennel et al., Cancer Res. 41, 3465-3470 (1981); Kennel et al., J. Biol. Chem. 264, 15515-15521 (1989)]. Finally, antibody GB36 was raised in mice against microvilli preparations of human placenta. The antibody recognizes cell surface proteins on human carcinoma cells [Hsi et al., Placenta, 8, 209-217 (1987)]. It was suggested in this reference that the protein (.alpha.6/.beta.1 integrin) may play a role in maintenance of cell polarity, however no functional assays were performed.
Now it has been surprisingly found on the basis of the specific ability of the monoclonal antibodies of the present invention to inhibit metastasis that a further molecule, namely the antigen recognized by these monoclonal antibodies, which is known as .alpha.6-integrin [for a review see Hemler in Annu. Rev. Immunol. 8, 365-400 (1990)], is probably the most important endothelial adhesion molecule to be involved in metastasis.
The antibody of the present invention does not interfere with the binding of cells to laminin but blocks cell-cell interaction of melanoma cells with vascular endothelial cells. This finding strongly suggests that the antibody of the present invention recognizes a so far unknown binding domain on the .alpha.6 integrin chain-clearly different from the binding sites of GoH3, 135-13C or GB36, by the fact that GoH3 cross-reacts with skin homing lymphocytes in sheep whereas the antibody of the present invention does not.